J . Phys. Chem. 1987, 91, 6620-6624
6620
Decay Kinetics of a Trlplet-Triplet Fluorescence System. The Mesttylylene Biradlcal V. Lejeune, A. Despres, B. Fourmann, 0. Benoist d’Azy, and E. Migirdicyan* Laboratoire de Photophysique Molbculaire du C.N.R.S.,tBtitiment 213, Universitb Paris-Sud, 91405 Orsay Cedex, France (Received: July 13, 1987)
Fluorescence decays of mesitylylene-hlo and -dLo(M-hIoand M-dIo)biradicals and mesityl-hll and -dll monoradicals are measured between 10 and 77 K. These fragments are generated “in situ” by UV photolysis of mesitylene dispersed in Shpolskii matrices or in glasses. While the fluorescence decay is a,single exponential for monoradicals, it is a sum of two exponential functions for biradicals in the whole temperature range between 10 and 77 K. The lifetimes of the two components are 30 f 6 and 195 f 15 ns for M-hIoand 50 f 10 and 265 f 20 ns for M-dlo. These lifetimes are independent of variations in excitation and observation wavelengths, temperatures below 40 K, and local environmental conditions. They are, however, significantly altered in the presence of a 4 5 0 4 magnetic field. One possible interpretation of these results is to attribute the biexponential decay to the fluorescencefrom two individual sublevels of the lowest excited triplet level which is the emitting state of mesitylylene biradical. This requires that the radiative process from the triplet sublevels be faster than the spin-lattice relaxation between the different sublevels. This condition is probably satisfied for the 50-300-11s fluorescence of mesitylylene biradicals.
Introduction m-Xylylene (m-quinodimethane), the prototype of aromatic non-Kekul6 biradicals, has been widely investigated by theoretical, chemical, and spectroscopic In fluid medium, this fragment can be photolytically generated as a short-lived intermediate and detected by means of its subsequent dimerization reaction or by trapping with conjugated diene^.^ When produced by photolysis in low-temperature matrices, m-xylylene can be observed directly by EPR3q4and optical spectroscopy.5,6 Biradicals have two degenerate or nearly degenerate nonbonding molecular orbitals (NBMOs) containing a total of two electrons. There are four zeroth-order configurations. The two configurations with single occupancy of each NBMO can be distinguished by their spin as “open-shell” singlets and triplets. The two configurations with double occupancy of one or the other of the NBMOs are mixed to produce two ‘closed-shell” singlets. A question then arises concerning the ground-state multiplicity. Semiempirical,6-’o nonempirical,” and ab initio’, molecular orbital calculations indicate that the ground state of planar mxylylene is triplet. This assignment has been confirmed by independent sets of EPR experiments involving different precursors. Wright and Platz4 have prepared m-xylylene by exhaustive photolysis of a bis-diazo derivative of m-xylene in ethanol at 77 K. Goodman and Berson3 subsequently generated the biradical in a 2-propanol glass at 77 K from a bicyclic hydrocarbon, by the Norrish type I1 photofragmentation. Except for an impurity pattern unique to each experiment, the six-line EPR signal of m-xylylene in its ground triplet state was found to be essentially the same in both laboratories. leading to the zero-field splitting parameters JDl/hc = 0.011 cm-I an; IEl/hc > 0.001 im-l, in agreement with theoretical expectation^.'^*^^ With the ground-state triplet identity established, interest has now turned to the excited-triplet-state manifold. Planar m-xylylene has C,,symmetry, and in the Mulliken axis convention (Figure l), the ground triplet state is B2. The two lowest excited triplets are predicted to occur in the visible absorption region with Bz and A I symmetries. These triplets are close in energy so that their ordering is theoretically uncertain. The ordering is also expected to be sensitive to methyl substitution on the ring, and this sensitivity may provide a systematic means to probe these close-lying electronic states. We have recently reportedI5J6 theoretical and spectroscopic studies on the whole series of methyl-substituted m-xylylenes. Fluorescence and excitation spectra of m-xylylene and its methylated derivatives in Shpolskii matrices at 5-10 K have been obtained by site-selective laser experiments. The fluorescence spectra can be analyzed by using the vibrational modes and 7 AssociC
5 1’Universite de Paris-Sud.
0022-3654/87/2091-6620$01.50/0
ground-state frequencies of parent molecules, but the activity of these modes depends on the number and on the position of the methyl groups. The dependence is particularly noticeable for mode 6b which derives together with mode 6a from the splitting of the benzene e2%mode 6 on lowering the symmetry from D6,, to C2&. While the totally symmetric mode 6a is observed in the fluorescence spectra of all the m-xylylenes, the nontotally symmetric mode 6b has significant one-quantum activity in the spectra of some biradicals and not in others. Such activity of mode 6b suggests the existence of a forbidden component in the first electronic transition of these biradicals. SCF-MO-CI calculations indicate that the first excited triplet is A, for all the biradicals except m-xylylene and 2-methyl-mxylylene which have B2 symmetry. Those two exceptions out of twelve biradicals are the two m-xylylenes without mode 6b activity in the fluorescence spectra. It is thus tempting to correlate mode 6b activity with an A, assignment of the emitting triplet, but whether this is correct and why it should be so have yet to be established. In order to explore the problem further, one can use information from fluorescence decay measurements. Provided that the decays are exponential, the measured lifetimes can be compared directly to the theoretical emission lifetimes of the lowest excited triplet states. The two possible transitions 2 3B2-1 3B2and 1 3A1-l 3Bz are both allowed on the basis of orbital symmetry, but the theoretical data listed in Table I of ref 16 indicate that the corresponding oscillator strengths are small. The calculated oscillator strengths of m-xylylene and its methylated derivatives are around respectively for B2-B2 and A,-B, tran5X and 30 X (1) Platz, M. S. In Diradicals; Borden, W. T., Ed.; Wiley-Interscience: New York, 1982; Chapter 5, pp 195-258 and references therein. (21 Berson. J. A. In Chemistry. ofFunctional G ~ O U DPatal. S : S . , RaoDoDort. “ .. . Z., ‘Eds., in press. (3) Goodman, J. L.; Berson, J. A. J. A m . Chem. SOC.1985, 107, 5409. (4) Wright, B.; Platz, M. S. J. A m . Chem. SOC.1983, 105, 628. (5) Migirdicyan, E. C. R. Hebd. Seances Acad. Sci. 1968, 226, 756. (6) Migirdicyan, E.; Baudet, J. J. A m . Chem. SOC.1975, 97, 7400. (7) Berthier, G.; Baudet, J.; Suard, M. Tetrahedron 1963, 19 (Suppl. 2), I.
(8) Baudet, J. J. Chim. Phys. Phys. Chim. Biol. 1971, 68, 191. (9) Dohnert, D.; Koutecky, J. J. A m . Chem. SOC.1980, 102, 1789. (10) Lahti, P. M.; Rossi, A. R.; Berson, J. A. J. Am. Chem. SOC.1985, 107, 2273. (1 1) Klein, D. J. Pure Appl. Chem. 1983, 55, 299. (12) Kato, S.; Morokuma, K.; Feller, D.; Davidson, E. R.; Borden, W. T. J . Am. Chem. SOC.1983, 105, 1791. (13) Rule, M.; Matlin, A. R.; Hilinski, E. F.; Dougherty, D. A,; Berson, J. A. J. Am. Chem. SOC.1979, 101, 5098. (14) Rule, M.; Matlin, A. R.; Seeger, D. E.; Hilinski, E. F.; Dougherty D. A,; Berson, J. A. Tetrahedron 1982, 38, 787. (15) Lejeune, V.; Despres, A,; Migirdicyan, E. J. Phys. Chem. 1984, 88, 2719. (16) Lejeune, V.; Despres, A.; Migirdicyan, E.; Baudet J.; Berthier, G. J . Am. Chem. SOC.1986, 108, 1853.
0 1987 American Chemical Society
Fluorescence Decays of Mesitylylene Biradical
The Journal of Physical Chemistry, Vol. 91, No. 27, 1987 6621
t.2
QO
D
4 890A
4
Figure 1. The C, symmetry m-xylylene and mesitylylene biradicals in the Mulliken axis convention.
sitions. It is seen that the fluorescences from biradicals having emitting levels of either B2 or AI symmetries are expected to have significantly different lifetimes, provided that the rates of radiationless transitions in competition with the fluorescence remain the same for the whole set of methylated biradicals. In this paper, we begin our decay measurements of methylated xylylenes with the meta biradical produced from 1,3,5-trimethylbenzene (mesitylene). In their perprotonated and perdeuteriated forms, we refer to these 3-methyl-m-xylylene (mesitylylene) biradicals as M-hIoand M-dlo. These biradicals have proven to have the most intense fluorescence in Shpolskii matrices and in glasses. It has turned out that the story is far more interesting than the simple picture presented above, since the fluorescence decays prove to be nonexponential.
Experimental Section n-Pentane and n-hexane solvents (Merck UVASOL) were used without further purification. Impurity luminescence could not be observed, when n-pentane and n-hexane frozen at 10-20 K were photolyzed and excited under conditions identical with those used for the mixed crystals. 3-Methylpentane (3-MP, Fluka) was purified by column chromatography on neutral aluminum oxide (Woelm). The purest samples commercially available of 1,3,5trimethylbenzene-h12(Fluka) and -d12(Merck Sharp and Dohme of Canada) were used without further purification. The mesitylylene biradicals and mesityl monoradicals were generated "in situ" by UV photolysis (unfiltered 200-W Osram to Hg lamp) of mesitylene (concentrations ranging from M) dispersed either in a glass of 3-MP or in polycrystals of n-pentane or n-hexane at 10-20 K. The samples were mounted on a holder inside an "Air liquide" cryostat which used helium gas as coolant. The excitation source was a home-built tunable dye laser pumped with a (Lambda Physik M 2000) nitrogen laser. Coumarin 440 dye from Exciton Co. was used in the dye laser. The pulse width at half-maximum is 6 ns. Fluorescence was analyzed with a T H R 1500 Jobin-Yvon spectrometer. The signal was detected by a X P 2020 photomultiplier (RTC) whose output was fed to a digital oscilloscope LeCroy 9400 equipped with signal-averaging WPOl software. Part of the light from the excitation radiation was directed on a photodiode to trigger the oscilloscope. When necessary, the laser pulses were reduced by neutral density filters in order to remain within the linear range. Emission from the fragments trapped in specific sites of glassy or crystalline matrices was excited with laser bandwidths of 4-6 cm-I. The decays were measured at selected spectrometer wavelengths with narrow but variable spectral widths. Some experiments were performed with carefully deaerated samples. The fluorescence decay curves were essentially identical with aerated as well as deaerated samples. The results described in this paper were obtained with nondegassed samples. Results The UV photolysis of mesitylene dispersed in Shpolskii matrices or in glasses at low temperatures gives rise to mesityl (3,5-dimethylbenzyl) monoradicals together with mesitylylene biradicals. These fragments result from the photodissociation of CH bonds on one or two different methyl groups of the parent molecule. The
Ca
Figure 2. Fluorescence spectra of photolyzed solutions of mesitylene-h12 in n-hexane at 18 K (A) and in 3-MP at 14 K (B and C). The emissions are excited through a Schott UG11 filter by the full emission of Hg lamp (A and B) and with a laser line at 4414 A (C). This radiation has been selected within the broad excitation band of the biradical at 600 cm-' from the 0,O band. Curve D corresponds to the fluorescence spectrum of m-xylyl monoradical produced by UV photolysis of m-xylene in npentane at 10 K.
mono- and biradicals can be distinguished by the energies and vibrational structures of their fluorescence and excitation spectra. In this paper, we are mainly concerned with the fluorescence decays of mesitylylene biradicals after excitation with visible laser radiation. Such excitation coincidently produces fluorescence from mesityl monoradicals whose electronic energy levels are lower than those of mesitylylene biradicals. A . Fluorescence Spectra. The fluorescence spectra of M-hlo biradicals together with those of mesityl-hll monoradicals either in a polycrystalline Shpolskii matrix (n-hexane) or in a glass (3-MP) are shown in curves A-C of Figure 2. The biradical emission has its 0,O band near 4500 A, and that of the monoradical is near 4900 A. The strong monoradical fluorescence overwhelms that of the biradical at wavelengths where the two emissions overlap. In the Shpolskii matrix, the spectra of biradicals are usually sharp and multisite since the fragments are dispersed in several crystallographically well-defined lattice sites. In the glass, however, the emissions are broad because the species occupy randomly distributed sites and therefore they have a distribution of electronic energy levels slightly shifted with respect to one another. These differences are illustrated by the biradical emission in curves A and B. By use of narrow-band laser excitation, it is possible to excite selectively the subset of biradicals having a specific electronic transition energy that matches the excitation energy. Such excitation produces sharp line fluorescence spectra in both types of matrices. In our previous report^'^,'^ we have already applied the site selection method to obtain quasi-line fluorescence from many m-xylylenes in Shpolskii matrices. Here we extend our study to glassy matrices. A comparison of spectra B and C shows a marked sharpening of fluorescence bands when the biradicals in a 3-MP glass are excited with a laser line. This is an application of the laser-induced fluorescence line-narrowing method that was first developed by Personov and c o - ~ o r k e r sfor ' ~ aromatic compounds (17) Personov, R. I. In Spectroscopy and Excitation Dynamics of Condensed Molecular Systems; North-Holland: Amsterdam, 1983; Chapter 10,
pp 555-619 and references therein.
Lejeune et al.
6622 The Journal of Physical Chemistry, Vol. 91, No. 27, 1987
II
FLUORESCENCE
Exc/rAr'oN
1
I
I
Figure 3. Site-selected fluorescence (B) and excitation (A) spectra of
M-dloisolated in n-pentane at 20 K. The emission is excited with a laser line at 4398 8,corresponding to the intense excitation band at 552 cm-I from the 0,O band. The laser-induced excitation spectrum is obtained with narrow-band observation of the fluorescence origin band.
in organic glasses. The sharpened structure confirms the attribution of biradical emission since the vibronic structure matches that observed previously (spectrum D, Figure 1, ref 15) for this species trapped in n-pentane a t 5-10 K. The attribution of the broad intense emission starting near 4900 A to mesityl-hll monoradical is based on the transition energy and the vibronic structure of its fluorescence. We do not yet know why this spectrum is so broad even for radicals in a Shpolskii matrix (spectrum A) since most methylated derivatives of the benzyl radical have quasi-line fluorescence spectra in such conditions. An example is provided by spectrum D in Figure 2 where we display emission from m-xylyl monoradical in n-pentane at 10 K. This well-resolved emission has been identified by the method of isodynamic molecules.I8 The broad intense emission present on curves A-C has been attributed to mesityl monoradical since it is the envelope of the vibronic bands of m-xylyl on curve D. This emission is found to be broad in several crystalline matrices such as frozen n-pentane, n-hexane, and mesitylene, and it remains broad even after annealing the sample by a warming to 77 K and then cooling to 10-20 K. Deuteriation of the precursor to produce perdeuteriated monoand biradicals gives spectra similar to those presented in Figure 2, but with a shift of about 130 cm-I to higher energies. The site-selected fluorescence and excitation spectra of M-dlobiradical in n-pentane at 20 K are displayed in Figure 3. The spectra of these perdeuteriated species are very similar in vibronic structure and relative band intensities to the corresponding spectra for M-hlo shown in ref 15. B. Fluorescence Decays. The fluorescence of biradicals and monoradicals is observed under the same experimental conditions since both fragments are prepared from the same parent molecule in the same sample. Fluorescence from the two species is always produced by a laser wavelength that is coincident with a strong biradical excitation band. Fluorescence from one species was isolated from that of the other by a spectrometer tuned to strong emission bands with a bandwidth of 2-20 cm-l depending on the fluorescence intensity. The fluorescence decays of M-d,o biradicals together with those of mesityl-d,, monoradicals trapped in n-hexane at 14 K are displayed on curves A and B of Figure 4. On this semilogarithmic scale, it appears clearly that the decay of the monoradical is exponential while that of the biradical is nonexponential. This is true for species produced from mesitylene-d12as well as from mesitylene-h12dispersed either in Shpolskii matrices or in a 3-MP glass. An analysis of the curves by the component stripping procedure indicates that the biradical decay is biexponential. The observed lifetimes of the two components and the corresponding fractions of the total number of emitted photons determined for M-hl0 and M-dlobiradicals in Shpolskii matrices are given in Table I. We observe that the lifetimes of the two components are significantly perturbed by deuterium substitution. (1 8) Leach, S.; Lopez-Campillo, A.; Lopez-Delgado, R.; Tomas-Magos, M. C.J . Phys., Colloq. 1967, 28, C3-147.
t
T I /4K
IJ0
1
200
I
400
1
600
000
t(nd
Figure 4. Fluorescence decays of M-dlo biradicals (A) and mesityl-dll monoradicals (B) in n-hexane at 14 K. Both decays are excited with a laser line at 4391 8,which corresponds to a strong excitation band of the biradical at 552 cm-I from the 0,O band. They are measured on the fluorescence origin band of the biradical at 4505.2 8, and of the monoradical at 4870 8,. LnI-
I
1
0
200
I
d
400
600
800
t(ns)
Figure 5. Fluorescence decays of M-hlo biradicals in two different sites of n-pentane at 22 K. The decays are excited with laser lines at 4406 8, (A) and at 4411 8, (B). They are measured on the fluorescence origin band at 4527.8 8, (A) and 4532.8 A (B). TABLE I: Observed Lifetimes of the Two Components and the Correspondiag F ~ c ~ ~ofo the M Total Number of Emitted Photons Determined from the Fluorescence Decays of M-B,,, and M-dlo Biradicnls in Shpolskii Matrices biradical
M-h,,, in n-pentane at'i2 K . M-d,, in n-hexane at 14 K
30 & 6
0.20
195 f 15
0.80
50 f 10
0.22
265 f 20
0.78
We have found that the decay characteristics of M-hlo and M-dlobiradicals are essentially independent of variations in exciting wavelengths, observation wavelengths, and band-passes. In addition, the biradical decay curves remain independent of temperature below 40 K. For higher temperatures, the overall fluorescence intensity decreases, such that the lifetime determinations become less accurate. Particularly noteworthy is, however, the persistence of the nonexponential behavior of the biradical fluorescence decays in the whole temperature range between 10 and I1 K. Let us now consider the dependence of the fluorescence decays on the specific site where a biradical is trapped. The insert in Figure 5 shows the structure of the fluorescence origin band of M-hlobiradicals in n-pentane at 22 K under broad-band excitation. This sharp and multisite structure results from the interaction of biradicals with their environment leading to a distribution of transition energies which is characteristic of the n-pentane Shpolskii matrix. If a unique lifetime were correlated with each transition energy, a similar distribution would be expected for the
Fluorescence Decays of Mesitylylene Biradical
The Journal of Physical Chemistry, Vol. 91, No. 27, 1987 6623
E(eV1
Figure 6. Fluorescence decays of M-dlo biradicals in n-hexane at 15 K in the absence (A) and in the presence (B) of a 450-G magnetic field. The decays are excited with a laser line at 4391 A and measured on the fluorescence origin band at 4505.2 A.
lifetimes which are susceptible to change with the environment. Figure 5 shows the selectively excited fluorescence decays from two discrete sets of M-hlo biradicals in n-pentane at 22 K, measured on the sharp origin bands at 4527.8 A (curve A) and at 4532.8 A (curve B) with a spectral width of 13 cm-I. The decay characteristics of curves A and B are similar, indicating that the relaxation law is independent of the lattice site of the Shpolskii matrix where the biradicals are trapped. Thus, there is no correlation between transition energies and decay components and therefore the nonexponential decay is not due to the superposition of exponential fluorescence decays from biradicals in different sites. The fluorescence decays of M-d,, biradicals in n-hexane at 15 K in the absence (curve A) and in the presence (curve B) of a 450-G magnetic field are presented in Figure 6. It is clear that the decay law is significantly altered by the magnetic field. In particular, the contribution of the long-lived component has drastically decreased with respect to that of the short-lived component. At times longer than 400 ns, the fluorescence intensity is too low to permit any evaluation of the contribution of the slow component. An upper estimation for the lifetime of the short-lived component gives about 70 ns. This value is obtained from the initial slope of the semilogarithmic plot presented in curve B of Figure 6.
Discussion In this paper we are primarily concerned with the decay kinetics of mesitylylene biradicals. It is difficult, however, to avoid comparing the fluorescence characteristics of mesityl monoradicals and mesitylylene biradicals since both species are always prepared together during photolysis. Furthermore, mono- and biradical emissions are always produced together when biradical fluorescence is excited. The emissions from both fragments differ fundamentally in two aspects. While the spectra of monoradicals are broad under all conditions, those of biradicals are always sharp under narrow-band excitation. The relaxation kinetics of both species are equally different. The fluorescence decays of monoradicals are exponential over three lifetimes, with lifetimes of 500 ns for mesityl-hll and 650 ns for mesityl-dll in Shpolskii matrices at 10-20 K. Exponential fluorescence decays have already been observed in several for benzyl radical and its methylated derivatives in organic glasses at 77 K. Our data on monoradicals fit the general pattern established by these previous studies. We are most concerned, however, with the nonexponential relaxation of biradicals. The data presented in Figures 4-6 show that the fluorescence decays of biradicals are biexponential. It is now (19) 6, 353. (20) (21) (22)
Bromberg, A.; Friedrich, D. M.; Albrecht, A. C. Chem. Phys. 1974, Laposa, J . D.; Morrison, V. Chem. Phys. Lett. 1974,28, 270. Okamura, T.; Obi, K.; Tanaka, I. Chem. Phys. Lett. 1974,26, 218. Okamura, T.; Tanaka, I. J . Phys. Chem. 1975,79, 2728.
Figure 7. Energy level diagram of mesitylylene biradical with calculated energies of both triplet and singlet states. The order of T ~ T,,,, and T , sublevels is arbitrary.
important to understand the cause of this time dependence. In the discussion below, we explore some of the possibilities. Several potential contributors to nonexponential decay are ruled out by the insensitivity of the decay curves to vsrious experimental conditions. As established in the Results section, the two decay components with such different lifetimes cannot arise from biradicals in two different sites of Shpolskii matrices. Contributions from emitting impurities can be eliminated since the nonexponential character is independent of excitation and fluorescence observation wavelengths. Nonfluorescent impurities can always affect the nonradiative decay of the biradical, but it is difficult to create a model whereby this would cause nonexponential decay. Energy transfer is possible between biradicals and the monoradicals always present in the system. In this transfer the biradical is the donor because its electronic energy levels are the higher and the monoradical is the acceptor. The transfer involves spin-allowed transitions in the donor (triplet-triplet) and in the acceptor (doublet-doublet), and thus it is allowed by spin selection rules for transfer by dipole-dipole i n t e r a ~ t i o n . Assuming ~~ a random distribution, F O r ~ t e has r ~ ~expressed the time evolution of the donor fluorescence intensity Z(t) in the presence of acceptor at concentration CAby I ( t ) = Io e x p [ - t / ~ - 2 y ( t / ~ ) ' / ~where ] T is the donor fluorescence lifetime. The quantity y is CA/CAo where CAo is the acceptor critical concentration. We have found that our observed decay curves of mesitylylene biradicals can never be fitted with Forster's expression. It seems therefore highly improbable that resonance energy transfer would be responsible for the nonexponential behavior of biradicals fluorescence decay. Biradical fluorescence is special in that it involves triplet states rather than singlet or doublet states more commonly encountered in spin-allowed transitions. Triplet emission is generally associated So phosphorescence, and a rich literature for this with TI provides examples of nonexponential emission decays. Although our biradical fluorescence is a TI To transition rather than the Tl So, we can explore the possibility that the nonexponential decays are the natural consequence of the triplet character of the emitting state. Specific examples of nonexponential decays occur in the phosphorescence of quinoxaline in durene at temperatures T lower than 4.2 K25and of pyrazine in cyclohexane at T lower than 10 K.26 Both decays became exponential either by increasing T o r by applying a magnetic field. These results have been explained
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(23) Fbrster, Th. Ann. Phys. 1948,2, 5 5 . (24) F h t e r , Th. Z.Naturforsch. A 1949,4, 321. (25) De Groot, M. S.; Hesselmann I . A. M.; van der Waals, J. H. Mol. Phys. i967,12, 259. (26) Hall, L.; Armstrong, A.; Moomaw, W.; El-Sayed, M. A. J . Chem. Phys. 1968,48, 1395.
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Lejeune et al.
The Journal of Physical Chemistry, Vol. 91, No. 27, 1987
by considering the competition between the spin-lattice relaxation and the radiative process from the three individual sublevels of the lowest triplet state. Let us now examine whether such a model can be used to explain the decay dynamics of the biradical fluorescence. Planar mesitylylene is a C2,symmetry species with molecular axes defined according to Mulliken’s convention (see Figure 1). An energy level diagram with the calculated energiesI6 of both singlet and triplet states is given in Figure 7. The calculations predict that mesitylylene has a triplet ground state To of B2 symmetry and a first excited triplet TI of A, symmetry.16 Three and possibly four singlet states are predictedI6 to lie below the emitting triplet Ti. The ground triplet state of mesitylylene is split into T,, T,,, and 7,components whose zero-field splittings (ZFS) D and E are close to the parameters IDl/hc = 0.01 1 cm-I and IEl/hc > 0.001 cm-’ determined for m-~ylylene.~.~ The ESR spectra obtained for both biradicals are found to be ~ i m i l a r .The ~ ZFS parameters have not been determined for the first excited triplet state Ti. In the C, point group, the three triplet spin functions T,, T,,, and T, belong to the b2, b,, and a2 irreducible representations, respectively, whereas the singlet spin function is of a, symmetry. The total symmetries of the To and the TI sublevels are given in Figure 7. Theory predicts that only T , (To) T, (TI), T,, (To) T~ (T,), and T, (To) T, (T,) transitions are electronically allowed and the corresponding transition moments are equal. In the absence of spin-orbit interaction and nonradiative decay, the fluorescence emitted from the T1 state is expected to be exponential with a lifetime of about 180 ns. The lifetime was calculated by using the approximate relationship T = 1.5/fu2 where u is the transition energy in cm-’ and f = 0.017, the oscillator strength calculated for the first electronic transition in mesitylylene.I6 By analogy with the quinoxaline and pyrazine model, the biexponential decay of biradicals would be consistent with fluorescence from independent TI sublevels at a rate faster than the rate of equilibration by spin-lattice relaxation. Furthermore, the decay must occur with distinct rates for each sublevel populated in the To TI excitation. A number of temperature-dependent processes are known to be responsible for the spin-lattice rela~ation.~’The direct process which prevails at low temperatures has a relaxation time which is inversely proportional to the absolute temperature T . The Raman process which is important at higher temperatures has In both cases, the sublevel a relaxation time proportional to T7. equilibration rate is expected to increase as T is raised, so that in quinoxaline and pyrazine nonexponential decays turn into exponential decays of an equilibrated sublevel population. The nonexponential character disappears above 4.2 K for quinoxaline and above 10 K for pyrazine. The fact that nonexponential decay persists at higher T for pyrazine is simply due to its shorter phosphorescence lifetime (1 8 ms for pyrazine, 237 ms for quinoxaline). If sublevel equilibration rates in our biradicals are like those of quinoxaline and pyrazine, we would expect our nonexponential decays to persist to much higher T . The persistence is purely due to the fast (50-300 ns) decay of the spin-allowed biradical T, To transition. Much faster sublevel equilibration rates would be required to destroy the independent character of the emitting
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(27) Waller, I. Z . Phys. 1932, 79, 370.
sublevels. Such a requirement would explain easily why the nonexponential character of our biradicals remains even at 77 K. We are unable to extend the studies to appreciably higher T because these unstable biradicals themselves begin to disappear. The second requirement of the model is that the T I sublevels decay with different rates. As mentioned above, calculations indicate that the moments of 7, (TO) T, (TI), T~ (TO) T~ (TI), T , (TI) transitions are equal in the absence of and T, (To) spin-orbit interaction. One can argue that this equality may disappear in better approximations. Spin-orbit coupling in mesitylylene biradicals should be very small, as it is the case for aromatic hydrocarbons. Salem and Rowland2* have shown that the matrix element of interaction between singlets and triplets is nonzero if there is some probability to find both electrons on the same NBMO in the singlet, provided that “the initial and final orbitals for the spin-flipping electron are as nearly orthogonal as possible”. This last condition is not satisfied in biradicals of the m-xylylene type where the two NBMOs have a pure x character, but it could be fulfilled if configuration interaction involves a significant contribution of (r orbitals. Symmetry rules indicate that spin-orbit coupling mixes singlets with triplet sublevels having the same symmetry. Among the three components of the ground triplet state, only 7x can couple with SI,S3, and S4 singlets having the same total symmetry A,. Similarly, in the TI state, only the T, sublevel can mix with the S2 and S5 singlets having the same total symmetry B2. Consequently, the 7, (To) T, (TI) transition which is perturbed by spin-orbit coupling can have a moment different from that of T,, (To) T,,(TI) and of 7, (To) 7, (TI) transitions, provided that the matrix element of this interaction is nonnegligible. By these arguments, one predicts, for m-xylylene biradicals, a biexponential radiative decay from Tl sublevels. Radiationless transitions such as intersystem crossing can also be responsible for the different decay rates from the TI sublevels. Here again spin-orbit coupling is involved, and the symmetry rules mentioned above indicate that, in the TI state, only the 7, sublevel can mix with the lower S2singlet having the same total symmetry B2. As a consequence, the rate of the nonradiative T , (TI) S2transition would be different from the nonradiative decay rates of the two other TI sublevels, leading to a biexponential fluorescence decay. The quinoxaline-pyrazine model remains, therefore, the most probable account of the phenomenon among the various possibilities that we have considered. This is supported by the sensitivity of the fluorescence decay curves of biradicals to the application of a magnetic field, as shown in Figure 6. A more detailed study on the dependence of the lifetimes on the field strength is currently in progress.
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Acknowledgment. We have greatly benefited from discussions with Professor J. H. van der Waals; we are indebted to him for his suggestion of a spin polarization mechanism. We are grateful to Dr. W. Siebrand for his interest in our work and for the gift of perdeuteriated mesitylene. We thank Dr. G. Berthier for his friendly advice on the theoretical background of our work. Registry No. M-hlo, 57384-02-8; M-dlo, 111437-27-5; mesityl-hIl, 19121-63-2; mesityl-d,,, 111437-28-6. (28) Salem, L.; Rowland, C . Angew. Chem., Int. Ed. Engl. 1972, 2, 92.
